1. Field of the Invention
This invention relates to an improvement of a noncontact type tonometer including a fluid discharging means for discharging a fluid towards the cornea of an eye to be tested, an alignment detecting means for detecting an alignment between the axis of a nozzle discharging a fluid discharged from the fluid discharging means and the vertex of the cornea and outputting an alignment signal, and a controlling means for determining whether or not the alignment has been completed upon receipt of the alignment signal from the alignment detecting means and outputting an alignment completion signal when the completion of the alignment has been determined, a fluid being automatically discharged from the fluid discharging means based on the alignment completion signal.
2. Prior Art of the Invention
A conventional noncontact type tonometer is disclosed, for example, in Japanese patent publication 58-34128. This conventional noncontact type tonometer includes an apparatus as a fluid discharging means for discharging a fluid towards the cornea of an eye to be tested, an alignment detecting means for detecting a completion of an alignment between the axis of the nozzle of the fluid discharging apparatus and the vertex of the cornea, and a controlling means for determining whether or not the alignment has been completed upon receipt of the alignment signal from the alignment detecting means and outputting an alignment completion signal when the completion of the alignment has been determined.
The alignment detecting means is designed as such that infrared rays are projected towards the cornea from the axial direction of the afore-mentioned apparatus, the infrared rays reflected by the cornea in the axial direction is received by a photoelectrically inverting element, and an alignment signal corresponding to the amount of the reflected infrared rays received by the photoelectrically inverting element is output.
The controlling means is adapted to compare a signal value corresponding to the amount of the reflected infrared rays received by the photoelectrically inverting element with a preset signal value corresponding to the amount of light received by the photoelectrically inverting element when the vertex of the cornea is in alignment with the axis, and output the alignment completion signal when the differential signal value is less than a predetermined value. Accordingly, the controlling means outputs an alignment completion signal when the alignment between the axis of the apparatus and the vertex of the cornea has been completed and automatically projects an air pulse as a fluid towards the cornea of the eye to be tested from the afore-mentioned apparatus. The pressure of the eye to be tested is measured based on the time from when the air pulse is projected towards the cornea from the axial direction until when the cornea is caused to be applanation.
However, in the conventional noncontact type tonometer, sometimes it happens that the fluid discharging means starts discharging a fluid in spite of the fact that an alignment between the vertex of the cornea and the axis of the apparatus has not been completely performed. Namely, sometimes it happens that the patient looks aside or the patient slightly fluctuates the fixation of his sight during the alignment verification. In such a case as just mentioned, according to the conventional noncontact type tonometer, sometimes it happens that a completion of the alignment is determined by chance, an alignment completion signal is output from the controlling means, although it happens only in a moment, and the fluid discharging means starts discharging an air pulse as a fluid towards the cornea of the eye to be tested based on the alignment completion signal. As a result, such obtained eye pressure measuring value contains a measuring error or errors due to the patient's side looking, the patient's minor fluctuation of the fixation of his sight, or the like.
Furthermore, the conventional noncontact type tonometer has the additional disadvantage in that after an air pulse as a fluid had been discharged based on the alignment completion signal and the measurement of an eye pressure had been completed, when the alignment has been accidentally completed with regards to that particular eye to be tested, an additional air pulse is discharged following the preceding air pulse discharge. Particularly, in the case of an eye to be tested which tends to largely fluctuate the sight fixation, or in the case of a baby or a child who has only a small capability of fixing his sight and tends to look aside, since a completion and a non-completion of an alignment are repeated within an extremely short time, air pulses are successively discharged each time, which likely renders pain and/or an unpleasant feeling to the patient. In addition, since an additional air pulse is discharged following the preceding air pulse discharge before the cylinder of the afore-mentioned apparatus has intaken a desired amount of air, an error or errors are occur in eye pressure measuring value.